1. Field of the Invention
The invention relates to a system incorporating
(a) a medium which presents signal sequences which consist at least partly of binary signal sequences, and
(b) a device for generating a synchronization signal for synchronizing at least parts of said signal sequences (words, blocks), where
(c) a binary signal sequence contains a synchronization pattern which consists of (2N+1) groups of codes, each with a number of n (n&gt;2) bits, a number of (at the most N+1) groups having a code content which has at least a given minimum Hamming distance with respect to the code (codes) of the other group (groups).
(d) said device comprising a majority decision device which produces the synchronization signal by means of said codes on the basis of a majority decision (N+1 out of 2N+1).
2. Description of the Prior Art
A system of this kind is known from U.S. Pat. No. 4,001,693. The known system utilizes a synchronization pattern which notably consists of 9 bits, i.e. 3 groups of 3 bits. One bit of each group is applied, possibly after inversion (depending on the selected codes), to the majority decision device (also referred to hereinafter as majority device). In the majority device it is tested whether at least two of the three bits applied thereto have the same value. Subsequently, the same is done for a second bit of each group, and finally for the third bit of each group. If the results of the three decisions taken have the correct value (the total result is compared with a predetermined code), a synchronization signal is generated.
It is to be noted that the expression "given minimum Hamming distance" is a measure of difference between code contents which is based on the known Hamming code theory. A minimum distance implies that at least one bit of a code group differs from the code content of another group.
The described generating of a synchronization signal notably takes place in communication systems as appears from said U.S. patent. There are also other systems where such generating of a synchronization signal is of importance: memory systems where data are written on/in a storage medium and are notably read therefrom. In systems of this kind it is very important to know when given data become available to the users (i.e. further parts of a system). An introductory section (heading) of a signal sequence is followed by the data section, notably in the form of a data block which may be subdivided into data words. In order to enable discrimination of such a block or the words thereof, synchronization by means of the said synchronization signal for "at least parts of said signal sequences" is required.
In view of the importance of proper block/word synchronization, attempts have been made to find solutions to minimize the risk of errors in the generating of such a synchronization signal. Therefore, in the known system the synchronization pattern consists of 2N+1=3 groups of n=3 bits which are examined in the majority device.
A major source of errors is formed by disturbances which extend over a length of a plurality of bits in the media used in the relevant systems. These disturbances are often referred to as "bursts" and may be interference sources in the communication paths and notably local deformations of the storage material in the storage media (in the form of defects, degenerations, etc.). When such a burst occurs at the time or the area of a synchronization pattern, it may be that no synchronization signal is generated or even that a synchronization signal is generated at an incorrect instant. This may have serious consequences, notably in the case where the synchronization signal occurs at an incorrect instant (for example, it may then be that data are written in a location where data are already present which may not be erased).
As long as a burst is small enough, that is to say if its length does not exceed the bit length n=3 of the code in a group, and if no error bit is present in any other location in the synchronization pattern of the known system, this synchronization pattern can still cope with the burst. However, if there is a risk that a burst has a bit length&gt;n, faults may occur.